Efficient production of a containerboard to be used as fluting

ABSTRACT

There is provided a method of producing a containerboard having a geometric SCT index of 37.0-42.0 Nm/g when measured according to ISO 9895:2008, comprising the steps of: —providing a pulp having a Schopper-Riegler (SR) value of 15-19 when measured according to ISO 5267-1:1999, wherein at least 70% by dry weight of the pulp is NSSC pulp; —forming a web from the pulp; —pressing the web in a press section comprising a shoe press, wherein the shoe press comprises a shoe press belt having discontinuous grooves and the line load in the shoe press is in the range of 1400-2000 kN/m; —drying the web from the press section in a drying section to obtain said containerboard.

TECHNICAL FIELD

The invention relates to a method of producing a containerboard to beused as fluting.

BACKGROUND

Neutral Sulfite Semi-Chemical (NSSC) pulping is an old process that itis well known in the field of paper pulping and in use in many pulpmills around the world. One of the reasons for using NSSC pulping is thehigh yield.

In NSSC pulping, the cooking liquor comprises sulfite, such as Na₂SO₃ or(NH₄)₂SO₃ and a base, such as NaOH or Na₂CO₃. “Neutral” means that thepH of the NSSC cooking liquor is generally between 6 and 10. Normally,the cooking time is between 0.5 and 3 hours and the cooking temperatureis 160-185° C. The NSSC pulp comprises comparatively high amounts ofresidual lignin, such as 15-20%, which make the NSSC pulp stiff. TheNSSC pulping is “semi-chemical” in the sense that it comprisesmechanical treatment/grinding (after the chemical (cooking step)).

The NSSC pulp is for example used to produce containerboard that issubsequently corrugated to form the fluting of corrugated board.

Examples of mills using the NSSC pulping method are: BillerudKorsnäs'mills in Gruvön (PM 6) and Skärblacka (PM4), Sweden; Mondi SwiecieS.A.'s mill in Swiecie (PM 4), Poland; Mondi's (Powerflute's) mill inKoupio, Finland; Stora Enso Oyj's mill in Heinola, Finland (HeinolaFluting Mill); S.C. Celrom S.A.'s mill in Drobeta, Romania; PackagingCorp. of America's mills in Filer City (PM1, PM2 & PM3), Tomahawk (PM2 &PM4) and Wallula (PM2), United States; Ilim Group's mills (PM1 and PM3)in Korjazma, Russia; Permsky Karton's mill (PM2) in Perm, Russia;WestRock's mills in Longview (PM10) and Stevenson (PM1 & PM2), UnitedStates; International Paper's mills in Mansfield (PM2) and Pine Hill(PM2), United States; Georgia-Pacific LLC's mills in Big Island (PM1 &PM3) and Cedar Springs, United States; Cascades ContainerboardPackaging's mill in Trenton, Canada; Sappi's Tugela mill (PM2) in SouthAfrica; Lake Utopia Paper's mill in St. George, Canada; GraphicPackaging International's mill in West Monroe, United States; Greif BrosCorp's mill in Riverville, United States; Hood Container Corp's mill inNew Johnsonville, United States; and Sonoco's mill in Hartsville (PM10),United States.

EP3026173 discloses a method of producing a containerboard (to be usedas fluting) of increased SCT strength from pulp comprising NSSC pulp.According to the method, a web formed from the pulp comprising NSSC pulpis pressed in a shoe press, in which the line load is at least 1200kN/m.

Further, it is generally acknowledged in the prior art that the SCTstrength of NSSC-based containerboard to be used as fluting is increasedwhen the NSSC pulp is subjected to more refining. As an example, therebuild of PM4 in Swiecie in 2015 involved a rebuild of the refiningsystem to allow for more refining of the NSSC pulp in order to increaseSCT strength. For the same purpose, Billerud AB (now BillerudKorsnäs AB)increased the refining capacity in the NSSC mill in Gruvön, Sweden backin 2005. Powerflute's NSSC mill and Stora Enso's mill in Heinolaimproved their refining capacity in 2010 and 2011, respectively.

SUMMARY

The object of the present disclosure is to increase the efficiency ofthe method disclosed in EP3026173 while maintaining the SCT strength ofthe product at a high level.

In the context of the present disclosure, the increased efficiency maybe an increased energy efficiency (i.e. a reduced amount of energyconsumed for producing one tonne of paper) and/or a higher productivity(i.e. that a higher amount of paper can be produced by the paper machineused for the method).

To meet the above-mentioned objects, there is provided a method ofproducing of a containerboard having a geometric SCT index of 37.0-42.0Nm/g when measured according to ISO 9895:2008, comprising the steps of:

-   -   providing a pulp having a Schopper-Riegler (SR) value of 15-19        when measured according to ISO 5267-1:1999, wherein at least 70%        by dry weight of the pulp is NSSC pulp;    -   forming a web from the pulp;    -   pressing the web in a press section comprising a shoe press,        wherein the shoe press comprises a shoe press belt having        discontinuous grooves and the line load in the shoe press is in        the range of 1400-2000 kN/m;    -   drying the web from the press section in a drying section to        obtain said containerboard.

The method may also be described as follows:

A method of producing of a containerboard having a geometric SCT indexof 37.0-42.0 Nm/g when measured according to ISO 9895:2008, comprisingthe steps of:

-   -   forming a web from a pulp in a forming section comprising a head        box, wherein at least 70% by dry weight of the pulp is NSSC pulp        and wherein the pulp in the head box has a Schopper-Riegler (SR)        value of 15-19 when measured according to ISO 5267-1:1999;    -   pressing the web in a press section comprising a shoe press,        wherein the shoe press comprises a shoe press belt having        discontinuous grooves and the line load in the shoe press is in        the range of 1400-2000 kN/m;    -   drying the web from the press section in a drying section to        obtain said containerboard.

The above range for the Schopper-Riegler value of the pulp (15-19)reflects a very low degree of refining, which means low energyconsumption in that stage of the papermaking process. The inventor hasalso shown that a reduction of the refining energy results in that lessenergy (i.e. less steam) is needed for drying the paper web formed fromthe pulp in the drying section. Surprisingly, the inventors have foundthat the low degree of refining is not associated with an impaired SCTstrength when the production is carried out according to the presentdisclosure.

Regarding the pressing of the web in the shoe press, the inventors havefound that increasing the line load to 1400 kN/m or more generally doesnot increase dewatering of the web unless the right type of shoe pressbelt is selected. When the right belt is selected, however, such anincrease of the line load effectively reduces the amount of steam neededto dry the paper web in the downstream drying section.

The present disclosure can thus make the production of NSSC-basedcontainerboard more energy efficient, which is of particular interestsince the yield of the NSSC pulping process is so high that theremaining biofuel (that is combusted in the recovery boiler) isinsufficient for the papermaking process. Consequently, NSSC mills aredependent on external energy. If the NSSC mill is not integrated with anadjacent sulphate mill that produces a surplus of biofuel, it may benecessary to use fossil fuel as the external energy. Examples of suchnon-integrated NSSC mills are the Heinola Fluting Mill and thePowerflute Mill. Further, the speed of many papermaking processes islimited by the drying capacity of the drying section. This limitation isa particular problem when containerboard of higher grammages isproduced. By providing a web that requires less drying energy in thedrying section, the present disclosure allows for higher speed and thushigher productivity in the making of NSSC-based containerboard.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a graph showing the shoe press line load and the resultingspecific steam consumption (medium pressure steam at about 8 bar) in thedrying section during the inventive trial described below. The shoepress line load was 1300 kN/m during a first period (“1^(st)”) and athird period (“3^(rd)”). During a second period (“2^(nd)”), the shoepress line load was 1700 kN/m.

DETAILED DESCRIPTION

There is thus provided a method of producing a containerboard having ageometric SCT index of 37.0-42.0 Nm/g. The containerboard is intendedfor use as fluting (i.e. corrugated medium) in corrugated board. Asunderstood by the skilled person, the method is intended to be used on afull-scale paper machine, i.e. a machine adapted to produce at least50,000 tons of containerboard per year, normally at least 100,000 tonsof containerboard per year. The grammage of the containerboard may be inthe range of 100-240 g/m², such as 110-240 g/m², such as 120-240 g/m²,such as 120-200 g/m² or 140-230 g/m², such as 150-230 g/m². Grammage ismeasured according to ISO 536:2012.

Preferably, the geometric SCT index of the containerboard is 38.0-42.0Nm/g, such as 38.0-41.0 Nm/g.

To obtain the geometric SCT index, the compressive strength in themachine direction (MD) and the cross direction (CD) of thecontainerboard is first measured using a short-span compressive tester(SCT) measured according to ISO 9895:2008. To calculate the compressivestrength index, the compressive strength (N/m) is divided by thegrammage. The unit of the SCT index is thus Nm/g. The geometric SCTindex is calculated as the square root of the product of the SCT indexin MD and CD:

geometric SCT index=√(SCT index (MD)*SCT index (CD)).

The compressive strength is considered to be more important in CD thanin MD. The SCT index in the CD of the containerboard may for example beabove 28 Nm/g, such as at least 29 Nm/g. An upper limit for the SCTindex index may for example be 32 Nm/g.

The method comprises the step of:

-   -   providing a pulp having a Schopper-Riegler (SR) value of 15-19        when measured according to ISO 5267-1:1999, wherein at least 70%        by dry weight of the pulp is NSSC pulp.

In one embodiment, the SR value is 16-19. In another embodiment, the SRvalue is 15-18.

The SR value referred to above (and in the claims) is the SR value thatthe pulp has in the head box (i.e. the chamber from which the pulp iscaused to flow onto the wire of the wire section). To obtain this SRvalue, the pulp of the present disclosure may be subjected to refining,such as LC refining, between the NSSC pulping process and the head box.

Preferably, at least 80% by dry weight of the pulp is NSSC pulp. In oneexample, at least 85% or 88% by dry weight of the pulp is NSSC pulp.

The NSSC pulp may comprise hardwood NSSC pulp. Preferably, at least 70%by dry weight of the NSSC pulp is hardwood NSSC pulp, such as birch NSSCpulp. In one example, at least 80% by dry weight of the NSSC pulp ishardwood NSSC pulp, such as birch NSSC pulp.

It has been reported in the prior art that a relatively low yield of theNSSC pulp is needed to obtain a large increase in strength. The resultspresented in the Examples section are however obtained using a NSSC pulpof a relatively high yield, more precisely about 82%. The yield of theNSSC pulp of the present disclosure may thus be 75%-85%, preferably79%-85%, such as 80-84%.

In addition to the NSSC pulp, the pulp may comprise recycled fibers,reject pulp and/or clippings (typically corrugated box plant clippings).The above-mentioned LC refining may be carried out before or aftermixing with such other fibres.

“NSSC pulp” is obtained from “NSSC pulping”, which in turn is defined inthe background section. The NSSC pulp of the present disclosure may forexample be sodium-based NSSC pulp, which means that the cooking liquorof the NSSC cook comprised Na₂SO₃.

The method further comprises the step of:

-   -   forming a web from the pulp (that has a SR value of 15-19),        typically in a wire section (as conventional in paper making).

In the method, the head box consistency may for example be 0.50%-1.20%,such as 0.80%-1.20%, such as 0.90%-1.15%. The higher consistencies areparticularly relevant when a large proportion (e.g. at least 80%) of theNSSC pulp is derived from hardwood.

In one embodiment, the head box consistency is 1.05%-1.20% and thegrammage of the containerboard is 140-240 g/m², such as 140-240 g/m².

The method further comprises the step of:

-   -   pressing the web in a press section comprising a shoe press.

The nip length in the shoe press may for example be 200-330 mm, such as250-300 mm. The shoe press is typically a double felted shoe press.

The shoe press comprises a shoe press belt having discontinuous grooves.Such a shoe press belt is commercially available. One example is theBlackBelt G DG marketed by Valmet. Another example is Valmet Black BeltH DG marketed by Valmet. The hardness of the elastomer in the shoe pressbelt may for example be 93-95 Shore A.

The line load in the shoe press is in the range of 1400-2000 kN/m,preferably 1500-2000 kN/m, such as 1600-2000 kN/m, such as 1600-1800kN/m.

In the shoe press, the web may be subjected to a press impulse of105-280 kPa*s, such as 105-190 kPa*s, such as 105-135 kPa*s. The highestpress impulses are obtained when the line load is relatively high andthe web speed is relatively low (because of a high grammage of theproduced containerboard).

The press section may comprise another press arranged upstream the shoepress. The other press is typically double felted and may for example bea jumbo press or a shoe press. The line load of the other press may be100-300 kN/m, such as 150-250 kN/m, in particular in case of a jumbopress.

Finally, the method further comprises the step of:

-   -   drying the web from the press section in a drying section to        obtain said containerboard. As explained above, the method of        the present disclosure facilitates a reduced steam consumption        in the drying section and/or production at a relatively high        speed, such as 700-850 m/min or even 750-850 m/min. When the        method of the present disclosure is carried out, the steam        consumption in the drying section may be as low as 1.20-1.35        tonne steam per tonne paper. This steam is normally “medium        pressure steam”, which refers to steam having a pressure of 6-11        bar, such as 6-10 bar, typically about 8 bar.

EXAMPLES First Reference Trial

Over a first period of about three months, NSSC containerboard (intendedfor fluting) of various grammages was produced in the Gruvön mill,Sweden. Data from the production is presented in table 1. The pulp was amixture of about 90% NSSC pulp from birch and about 10% kraft pulp. Thepulp was refined. The resulting SR numbers are presented in table 1. Thepress section had two double felted nips; first a jumbo press and then ashoe press with a shoe press belt having blind drilled holes. The lineload of the jumbo press was 180 kN/m.

Over a second period of about three months, NSSC containerboard(intended for fluting) of the same grammages was produced in the Gruvönmill, Sweden. Data from the production is presented in table 2. Again,the pulp was a mixture of about 90% NSSC pulp from birch and about 10%kraft pulp and it was refined. The resulting SR numbers are presented intable 2. The press section was the same as during the first periodexcept that another shoe press belt was used. This new shoe press belthad discontinuous grooves. The characteristics of the grooves were:

width: 1.30 mm;land: 1.80 mm;depth: 1.30 mm;open area: 38%;void volume: 460 ml/m².

From the data of tables 1 and 2, the change of shoe press belt does notappear to have had any particular effect. It is notable that thespecific steam consumption was not reduced during the second perioddespite that the average SR number was slightly lower than during thefirst period. It is also notable that the average geometric SCT indexwas almost the same during the second period as during the first period.

TABLE 1 First reference trial, first period (shoe press belt havingblind drilled holes). Head box Line load SCT Grammage SR slice lip Headbox shoe press SSC SCT Geo (kg/m²) number (mm) consistency (%) (kN/m)(ton/ton) Geo index 0.110 17.9 15.7 0.92 1 300 1.44 4.3 39.1 0.120 18.616 1.01 1 300 1.46 4.7 39.2 0.130 19.2 16.2 1.03 1 300 1.5 5.1 39.20.140 19.9 16.7 1.07 1 300 1.53 5.4 38.6 0.150 19.4 17.3 1.08 1 296 1.535.8 38.7 0.160 19.4 17.9 1.11 1 300 1.53 6.1 38.1 0.175 19.2 20.4 1.06 1300 1.54 6.9 39.4 Average 19.1 1 299 1.504 38.9 “SSC” means specificsteam consumption. The SR numbers were measured online.

TABLE 2 First reference trial, second period (shoe press belt havingdiscontinuous grooves). Head box Line load SCT Grammage SR slice lipHead box shoe press SSC SCT Geo (kg/m²) number (mm) consistency (%)(kN/m) (ton/ton) Geo index 0.110 19 15.8 0.92 1 300 1.5 4.3 39.1 0.12018.8 16.1 1.00 1 275 1.49 4.7 39.2 0.130 19 16.4 1.02 1 289 1.49 5 38.50.140 18.3 16.6 1.08 1 341 1.53 5.4 38.6 0.150 18 17.1 1.09 1 309 1.565.7 38.0 0.160 18 17.9 1.11 1 325 1.54 6.2 38.8 0.175 16.8 19.5 1.11 1300 1.48 6.9 39.4 Average 18.3 1 306 1.513 38.8 “SSC” means specificsteam consumption. The SR numbers were measured online.

Second Reference Trial

NSSC containerboard (intended for fluting) having a grammage of 140 g/m²was produced in the Gruvön mill, Sweden. The pulp was a mixture of about90% NSSC pulp from birch and about 10% kraft pulp. The pulp was refined.

The resulting SR numbers are presented in table 3 along with other datafrom the production. The press section had two double felted nips; firsta jumbo press (line load=180 kN/m) and then a shoe press with a shoepress belt having blind drilled holes. During a first period of about 28hours, the shoe press line load was 1500 kN/m. During a second period ofabout 34 hours, the shoe press line load was again 1300 kN/m. Data fromthe production in presented in table 3.

The data in table 3 show that the increased line load of 1500 kN/m didnot reduce the specific steam consumption.

TABLE 3 Second reference trial (grammage = 140 g/m², shoe press belthaving blind drilled holes). Average Head box Head box Line load AverageSR slice lip consis- shoe press SSC Period number (mm) tency (%) (kN/m)(ton/ton) First 15.9 17.0 1.05 1500 1.54 Second 16.1 17.1 1.05 1300 1.52“SSC” means specific steam consumption. The SR numbers were measuredonline.

Inventive Trial

NSSC containerboard (intended for fluting) having a grammage of 160 g/m²was produced in the Gruvön mill, Sweden. The pulp was a mixture of about90% NSSC pulp from birch and about 10% kraft pulp. The pulp was refined.The resulting SR numbers are presented in table 4 along with other datafrom the production. The press section had two double felted nips; firsta jumbo press (line load=180 kN/m) and then a shoe press having a shoepress belt having discontinuous grooves with the characteristicsdescribed above under the first reference trial. During a first periodof about 2.5 hours, the shoe press line load was 1300 kN/m. The shoepress line load was then gradually increased and during a second periodof about two hours, the shoe press line load was 1700 kN/m. The shoepress line load was then gradually decreased and during a third periodof about one hour, the shoe press line load was again 1300 kN/m. Datafrom the production is presented in table 4 (see also FIG. 1).

The data in table 4 and FIG. 1 show that in case of a shoe press belthaving discontinuous grooves, increasing the line load above 1300 kN/m(e.g. to 1700 kN/m) reduced the specific steam consumption considerably.On average, the specific steam consumption was 5% lower at 1700 kN/mthan at 1300 kN/m.

Further, the average geometric SCT index of the containerboard producedduring the second period was 38.3 Nm/g. During the same period, the SCTindex in the cross direction was 29.4 Nm/g.

TABLE 4 Inventive trial (grammage = 160 g/m², shoe press belt havingdiscontinuous grooves). Average Head box Head box Average SR slice lipconsis- Line load shoe SSC Period number (mm) tency (%) press (kN/m)(ton/ton) First 17.2 18.1 1.10 1300 1.38 Second 17.3 18.0 1.11 1700 1.33Third 17.3 18.1 1.10 1300 1.41 “SSC” means specific steam consumption.The SR numbers were measured online.

1. A method of producing a containerboard having a geometric SCT indexof 37.0-42.0 Nm/g when measured according to ISO 9895:2008, comprisingthe steps of: providing a pulp having a Schopper-Riegler (SR) value of15-19 when measured according to ISO 5267-1:1999, wherein at least 70%by dry weight of the pulp is NSSC pulp; forming a web from the pulp;pressing the web in a press section comprising a shoe press, wherein theshoe press comprises a shoe press belt having discontinuous grooves andthe line load in the shoe press is in the range of 1400-2000 kN/m;drying the web from the press section in a drying section to obtain saidcontainerboard.
 2. The method according to claim 1, wherein the web issubjected to a press impulse of 105-190 kPa*s in the shoe press.
 3. Themethod according to claim 1, wherein the speed of the web is in therange of 700-850 m/min.
 4. The method according to claim 1, whereingrammage of the containerboard is in the range of 120-240 g/m², whenmeasured according to ISO 536:2012.
 5. The method according to claim 1,wherein line load in the shoe press is in the range of 1500-2000 kN/m.6. The method according to claim 1, wherein at least 80% by dry weightof the pulp is NSSC pulp.
 7. The method according to claim 1, wherein atleast 88% by dry weight of the pulp is NSSC pulp.
 8. The methodaccording to claim 1, wherein the yield of the NSSC pulp is 75-85% 9.The method according to claim 1, wherein the containerboard has ageometric SCT index of 38.0-42.0 Nm.
 10. The method according to anyclaim 1, wherein at least 70% by dry weight of the NSSC pulp is hardwoodNSSC pulp.
 11. The method according to claim 1, wherein at least 80% bydry weight of the NSSC pulp is hardwood NSSC pulp
 12. The methodaccording to claim 1, wherein the steam consumption in the dryingsection is in the range of 1.20-1.35 ton steam per ton paper.
 13. Themethod according to claim 1, wherein the press section comprises anotherpress arranged upstream the shoe press.
 14. The method according toclaim 1, wherein the consistency of the pulp in a head box used forforming the web is 0.50%-1.20%.
 15. The method according to claim 10,wherein the consistency of the pulp in a head box used for forming theweb is 0.80%-1.20%.
 16. The method of claim 10, wherein the hardwoodNSSC pulp is birch NSSC pulp.
 17. The method of claim 13, wherein thepress arranged upstream of the shoe press is a jumbo press.
 18. Themethod according to claim 5, wherein line load in the shoe press is inthe range of 1600-2000 kN/m.
 19. The method according to claim 8,wherein the yield of the NSSC pulp is 79-85%.
 20. The method accordingto claim 9, wherein the containerboard has a geometric SCT index of39.0-42.0.